Condenser microphone



July 27, 1948.

N. E. BREWER 2,445,821

CONDENSER MICROPHONE Filed March 29, I944 Patented July 27, 1948.

UNITED STATES lTENT OFFICE:

CONDENSER MICROPHONE Noble E. Brewer, Abilene, Kans.

Application March 29, 1944, Serial No. 528,591

This invention appertains to'improvements in condenser microphones, and has among its several objects to provide certain refinements in structure and arrangement of parts that will effeet a greater fidelity and faithfulness in tone reception, throughout the range of sound vibrations impressed on the diaphragm, and which cure a sufficient output, a rather high polarizing voltageis applied between the condenser plates; voltages from 90 to 180 volts being commonly employed for the purpose. Heretofore, the microphone has been made with a heavy back plate fronted by a thin diaphragm that is tightly stretched to eliminate resonance. The diaphragm is usually a thousandth of an inch thick, with that amount of spacing between the vibrating member and the back plate. When sound waves are impressed on the diaphragm its oscillators movement varies the electro-static capacity between the condenser plates, which impresses an A. C. voltage on the D. C. polarizing voltage. This A. C. voltage is then transferred to the grid of the first audio amplifier through a small blocking condenser which isolates the polarizing voltage from the grid of the first audio tube.- In order to augment the minutes currents up to a desired level, a pre-amplifier is necessary and this is usually mounted in the microphone head. Coupled with this disadvantage, condenser microphones have a very high impedance which requires that they be isolated from both R. F. andA. C; fields. Also, weather conditions, such as humidity and barometric pressure, effect the response characteristics of practically all electro-static devices. Cavity resonance, structural and resonance peaks tend to alter the fidelity of many types of condenser microphones, especially those of inferior manufacture. In the condenser microphone em-- ploying the back plate fronted by the diaphragm,

two serious drawbacks have been noted, first, distortion resulting from increased capacity be-- tween the two condenser elements caused by blasting effects on the diaphragm when it was 7 Claims. (01. 179-411) actuated by high sound peaks, and, second, shorting of the polarizin voltage by contact of the diaphragm with the back plate, due to abnormalities in diaphragm vibration, also caused by high sound peaks and loud noises. This was remedied by increasing the spacing between the condenser elements, but with a resulting loss in signal strength and, consequently, a lowering in over-all efiiciency.

The instant invention overcomes these drawbacks, by changing the relative positions of the condenser elements, i. e., by placing the diaphragm in back of the fixed element or plate and perforating the latter to permit the sound vibrations to pass therethrough to the diaphragm; By this arrangement, sound waves impressed on the diaphragm cause it to move away from the fixed element or plate, with the result that all possibility of shorting of the applied polarizing voltage is eliminated. Also, a greater clarity in tone reception is to be had, by reason of the elec-' certain new and useful combination, construc? tion, and arrangement of parts, as will be here]- inafter more fully described, set forth in the appended claims, and illustrated in the accompanying drawing, in which:

Figure l is a front elevation of the microphone head and the upper end of a pre-amplifier case on which it is mounted;

Figure 2 is a vertical longitudinal section, taken through the line 2-2 of Figure 1.

Figure 3 is a vertical transverse section, taken through the line 33 of Figure 2;

Figure 4 is a sectional view similar to that of Figure 3, but taken through the line 4-4 of Figure 2;

Figure 5 is another sectional view similar; to

3 that of Figures 3 and 4, but taken through the line 55 of Figure 2;

Figure 6 is a face view of the front or fixed condenser plate, showing one arrangement of perforations therein;

Figure '7 is a view similar to Figure 6, but showing another arrangement of perforations in the front or fixed condenser plate;

Figure 8 is a sectional detail, showing the relative positions of the movable and fixed con denser elements, when at rest;

Figure 9 is a view similar to Figure 8, but showing the movement of the movable condenser element, or diaphragm, when it is subjected to the pressure of a sound wave 4|; and,

Figure 10 is a diagram of the circuit coupling the condenser microphone head with the first stage of resistance coupled amplification of a condenser microphone pre-amplifier.

Referring to the drawing, wherein like characters of reference denote corresponding parts in the several views, the improved microphone head, as it is exemplified therein, is comprised in a hollow case M that has its inner end open and fitted into an opening formed in the upper part of the front wall of a substantially larger case H in which a ma-amplifier (Figure 10) is to be housed. The case M has its open top provided with a removable closure or lid i2 and its back wall perforated, as at B3, in line with the axial center of the case M. The front end of the case M is partially closed by a wall portion I5 that surrounds a circular opening I6, for the ingress therethrough of voice, music, or other sound vibrations or waves, to be amplified and transmitted. Spaced about the outer side of the case I4 is a series of lugs H which are apertured to receive suitable fastenings, such as the bolts l8, for their sccurement to the front wall of the case M.

Fitted within the case M, and abutted against the inner side of the front wall portion I5, is a fiat annular member or ring l9, which has its open center made to a slightly greater area than that of the opening l6 and covered with a taut disc of a woven fabric 26, preferabl vegetable fiber. This covering prevents ingress through the opening 5 of dust, lint, insects, etc., and also functions as a filter for the sound vibrations or waves passing therethrough to the interior of the case.|4. Abutted against the inner face of the annular member or ring I3, is a non-metallic plate or disc 2|, of an insulating material, such as Bakelite, which is formed with a centrally disposed group of apertures 22 and an annular group of apertures 23, surrounding the central group. Also, four other apertures 24 (Figures 2 and 5) are equidistantly spaced about the central group 22, and between certain of the apertures of the outer group 23, for the insertion inwardly therethrough of screws 25, which are preferably flat head, 4/40, brass machine screws.

Mounted on the inner ends of the screws 25, and spaced uniformly from the inner side of the non-metallic plate or disc 2| by spacer sleeves 26, also of brass, is a plate 21, constituting the fixed condenser element of the improved microphone. This condenser plate 21 is in the form of a disc of brass and has an area corresponding to a central area of the plate or disc 2| that is bounded by the outermost of the apertures in the annular group 23. Also, the condenser plate 21 is provided with a centered group of aper tures 28 (Figure 4), which correspond to and are generally alined with the apertures of the central group 22, in the plate or disc 2|, the apertures of each of the groups 22, 28 being arranged in closel spaced relation, so that sound waves will pass directly from the apertures 22 to and through the apertures 28, without any disturbing interference.

Slidably fitted within the case M is a pair of matched annular members or flat rings 22 between which is :clamped a thin metallic disc 30, preferably of aluminum which constitutes the movable element or diaphragm of the condenser microphone, the rings and the diaphragm being secured together by rivets 29. The diaphragm 30 is supported by the rings 29 in back of the fixed plate 21 and in spaced relation thereto, the openings in the rings 29 having an area substantially equal to that of the opening IS, in the front wall of the case It, thus permitting of an adjustment .of the diaphragm relatively to the fixed plate 21, without any interference from the latter with respect to the clamp ring nearest thereto.

Interposed between the diaphragm carrier rings 29 and the inner side of the face plate 2|, is a spring compression ring 3|, the open center of which has an area substantially equal to that of the opening I6, in the front wall of the case I l, and, consequently, of the effective area of the diaphragm 30, so that there is no obstruction ofiered to the passage of the sound waves between the opening l6 and the diaphragm 30, through the respective groups of apertures 22 and 28, of the face plate 2| and the fixed condenser plate. 21.

To effect the adjustment of the diaphragm 30. relatively to the fixed condenser plate 21, the edge of the annular side wall 33, of a closure 34 for the open back end of the case M is inwardly beveled, as at 32, to provide a substantially feathered line of bearing on the opposed surfaces of the inner one of the clamp rings 29; the side wall 33 being arranged in screw-threaded engagement in that end of the, casing for turning move ments relatively to the case. The end wall of the closure 34 is provided with a centrally disposed opening 35, for the passage therethroughof the B plus and the B minus leads 36 and 33, from their points of connection with the fixed condenser plate 21 and the ring carrier frame 29, of the diaphragm 30, respectively; the leads passing from within the case M outwardly through the opening 35 and into the pre-amplifier case M, where, as is shown in Figure 10, the lead 36 is directl connected to the pre-amplifier circuit and the leads 38 to ground; the pre-amplifier preferably having at least two stages of resistance coupled amplification, of which one only is shown. Thus, when sound waves are impressed on the diaphragm 30 its oscillatory movement varies the electro-static capacity between it and thefixed condenser plate 21, which impresses an A. C. voltage on the D. C, polarizing voltage. This A. C. voltage is then transferred through a blocking condenser 39 (Figure 10) which isolates the polarizing voltages from the grid of the first audio amplifier tube 40.

In order to obtain a proper and accurate adjustment of the diaphragm 30 relatively to the fixed condenser plate 21, the leads 36 and 38 are first disconnected from the B plus and B minus power supply and are then connected to the terminals of an ohmmeter (not shown) for a continuity test. This test is made by now turning the back closure 34 slowly inward of the case I4, to move the diaphragm 30 toward the fixed condenser plate 21, the movement being opposed lished, the back closure 34 is slowly turned in the reverse direction and, sufficiently so, only to break the continuityreading on the ohmmeter and, consequently, the contact between the diaphragm and the fixed condenser plate 21, which spacesetting of the diaphragm 30 is best for high fidelity condenser microphone operation. Thepressure of the compression spring 3|, exerted,

against the opposed surface of the diaphragm ring carrier, is amply sufiicient to hold the parts in set position, following such adjustment. After adjustment, the ohmmeter is disconnected from the leads 36 and 38, and these are again connected to the B plus and B minus power supply, and pre-amplifier.

In order to obtain an equalization of the air in the back of the diaphragm 3D, in addition to the relief afforded by the group of holes l3 provided in the back wall of the pre-amplifier case H, as before stated, and the central opening 35, in the end wall of the closure 34, at the back of the microphone case 14, other holes 35 are provided in the latter wall, to that end. This equalizing of the air pressure at the back of the diaphragm 30, increases its sensitivity to sound vibrations and otherwise improves the quality of tonal reception.

In actual practice, it has been noted that the number and arrangement of holes or apertures provided in the fixed condenser plate 27 governs, in large measure, the frequency response, or sensitivity, of the diaphragm 30, e. g., when the number of holes or apertures occupies a total area of one-half or more of the plate, the volume re sponse of the diaphragm drops off nearly to zero. From this, it has been deduced that, in no case, should the combined hole or aperture area exceed more than one-third of the area of the plate. In Figure 4, the fixed condenser plate 21 is shown as having nine (9) apertures 28 arranged in the form of a cross and, in this instance, the frequency response of the diaphragm 30 is found to be fair, with a fair signal strength for volume also. In the modified fixed plate 21', sh'own in Figures 2 and 6, the nine (9) apertures are rearranged in closely grouped relation at the center of the plate, after the manner of the central group of apertures 22 of the face plate 2|, shown in Figures 2 and 5. Here, eight of the apertures are arranged in circular series about a single aperture located in the dead center of each of the plates 2| and 21'. With this arrangement of the apertures 28', in the plate 21, the frequency response of the diaphragm 30 is excellent and the sensitivity maximum, the response being particularly good for low audio frequency voice and sound vibrations. In the modified condenser plate 21", shown in Figure 7, twelve apertures 28" are drilled in circular series around the outer edge portion of the plate; with which arrangement, vibrations passing through the apertures 28" have a tendency to attenuate out a goodly part of the low frequency voice and sound vibrations, leaving the high audio frequencies to be reproduced by the diaphragm. The reason assigned to this fact, is that, when the voice and sound vibrations pass through the apertures 28', they strike the diaphragm 30 at its outer edge close to its point of support from the clamp rings 29, at which point it is stretch'ed the tightest, With the result that the diaphragm has a greater affinity for high audio frequencies.

Now, by reference to Figures 8 and 9, which show the relative positions of the condenser elements, 1. a, the diaphragm 3i) and the fixed plate 21, when the' diaphragm is set to vibrating, by the sound vibrations 4| (Figure 9), from its position of rest (Figure 8), it is obvious that any possible shorting of the electrical path through the diaphragm and plate, i. e., contact with one another, is prevented, since the greatest vibrating movement of the diaphragm is rearward, away from the fixed plate 21, than toward it, as was the case in the former arrangement of these condenser microphone parts. i

In the circuit diagram of Figure 10, showing the first stage of resistance coupled amplification of a condenser microphone pre-amplifier, using a triode tube 40, the fixed perforated front plate 21, of the microphone head, is impressed with a polarizing voltage when connected with B plus volts, it being also connected to the grid of the tube 40 through a fixed coupling condenser 39, while the diaphragm 30 is connected to both ground and the B minus side of the 180 volt, polarizing voltage.

Having thus fully described my invention, it is to be understood that various changes in design and minor details in construction and arrangement of parts, to which the improved condenser microphone is susceptible, may be resorted to, Without departing from the spirit of the inv ntion or its scope as claimed.

What I claim is:

1. In a condenser microphone head, a case open at its front and back ends, a perforated face plate engaged in the front opening of the case, a perforated fixed condenser plate positioned in back of and electrically insulated by the supporting perforated face plate, a diaphragm mounted in back of the fixed condenser plate, and a closure for the back open end of the case, said fixed condenser plate being perforated for the passage therethrough of sound vibrations received through the perforations in the face plate for impinging on the diaphragm.

2. The microphone head as in claim 1, with the fixed condenser plate supported from the inner side of the face plate. f

3. The microphone head as in claim 1, with an annular carrier for the diaphragm and means for adjusting the carrier frame for regulating the spacing of the diaphragm relatively to the fixed condenser plate.

4. The microphone head as in claim 1, with a resilient compression ring positioned about the fixed condenser plate and between the face plate and the diaphragm mountin to maintain the diaphragm required spaced relation to the fixed condenser plate.

5. The microphone as in claim 1, with the diaphragm mounted for sliding movement within the case and the closure also mounted for movement inwardly of the back open end of the case, the closure being operable to adjust the diaphragm relatively to the fixed condenser plate.

6. In a condenser microphone, the combination with a pre-amplifier case and a microphone head mounted in the front wall of the case, said microphone head comprising a case open at its front and back ends, a perforated face plate mounted in the front open end of the case, a

justably mounted in back of the fixed condenser plate, and a closure for the back open end of the microphone head case, the back Wall of the preamplifier case and the end wall of the closure having perforations through the same for equal-- izing the air pressure in back of the diaphragm.

7. The microphone headas in claim 1, With a filter covering the front side of said face plate, said filter being composed of Woven vegetable fiber so intersticed as to allow sound vibrations to pass 10 through the mesh.

NOBLE E. BREWER.

REFERENCES CITED The following references are of record in the file of this patent:

Number Number UNITED STATES PATENTS Name Date Crandall May 29, 1923 Massolle et a1. Aug. 18, 1925 Vogt Nov. 18, 1930 Vogt -1 Aug. 4, 1931 Wilson July 6, 1937 Olson Nov. 10,- 1942 Rettinger Apr. 11, 1944 FOREIGN PATENTS Country Date GreatBritain Sept. 23, 1926 Great Britain Nov. 30, 1934 

